Broad band objective having improved lateral color performance

ABSTRACT

A system and method for inspection is disclosed. The design generally employs as many as four design principles, including employing at least one lens from a relatively low dispersion glass, at least one additional lens from an additional material different from the relatively low dispersion glass, generally matching the relatively low dispersion properties of the relatively low dispersion glass. The design also may include at least one further lens from a further material different from and exhibiting a significantly different dispersion power from the relatively low dispersion glass and the additional material. Finally, the design may include lenses positioned to insert a significant amount of color within the objective, a gap, and additional lenses, the gap and additional lenses serving to cancel the color inserted.

This application is a continuation of U.S. patent application Ser. No.11/245,591, entitled “Broad Band Objective Having Improved Lateral ColorPerformance,” inventors Yung-Ho Chuang et al., filed Oct. 5, 2005, whichclaims the benefit of U.S. Provisional Patent Application 60/683,886,“Broad band Objective Having Improved Lateral Color Performance,”inventors Yung-Ho Chuang et al., filed May 23, 2005, both of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of optical imaging,and more specifically to optical systems used for microscopic imaging,inspection, metrology and lithography applications.

2. Description of the Related Art

Many optical systems have the ability to inspect or image features onthe surface of a specimen, such as inspecting defects on a semiconductorwafer or photomask, or alternately examining a biological specimen on aslide. Microscopes have been used in various imaging situations,including biology, metrology, semiconductor inspection., and othercomplex inspection applications where high resolution images of smallareas and/or features are desired.

When inspecting features on a specimen surface, a broadband objectiveexhibiting highly accurate and near perfect optical performance isparticularly desirable. This is particularly true for alignment systemssuch as those used for photomask alignment. However, all standardobjectives are typically limited at the edge of the field by lateralcolor.

In general, lateral color represents a difference between differentwavelengths of light, such as blue light and red light. FIG. 1 shows theprincipal ray of an optical system formed from a positive lens 101. Bluelight ray 102 in this simple arrangement tends to be more stronglyrefracted than red light ray 103 due to the refractive index changingwith wavelength. The difference in position of the rays is the lateralcolor 104, a standard definition based on the principal ray position.Note that the standard definition for lateral color does not include theeffects of monochromatic aberrations such as coma and chromaticvariation of coma. These aberrations can move the image centroid awayfrom the principal ray location. Monochromatic aberrations and thechromatic variation of the monochromatic aberrations produce anadditional contribution to lateral color when based on such an imagecentroid definition.

Lateral color according to this centroid definition can yield problemswhen inspecting under precise conditions. In other words, lateral colorcauses a shift in the image centroid for different colors. The centroidshift can adversely impact the image, particularly at the edge of thefield. Lateral color can also limit the accuracy obtained when usingoptics in metrology applications.

Some new methods for high order color correction can reduce the effectsof lateral color and the aforementioned chromatic centroid shift to lessthan 1 nanometer (nm) over a very broad wavelength range, generallyimproving the image at the edge of the field. As centroid shift can varyfor different colors, a design where the centroid shift for all colorsis reasonably uniform can be highly desirable, particularly whenchanging the focus position for the image.

It would therefore be beneficial to provide a system for use inmicroscopy that overcomes the foregoing lateral color drawbacks presentin previously known systems and provide an optical inspection ormetrology system design having improved functionality over devicesexhibiting those negative aspects described herein.

SUMMARY OF THE INVENTION

According to a first aspect of the present design, there is provided ahigh order color correction specimen inspection or metrology apparatus.The apparatus comprises at least one lens constructed from a relativelylow dispersion glass, at least one additional lens constructed from anadditional material different from the relatively low dispersion glassof the at least one lens. The apparatus further comprises at least onefurther lens constructed from a further material different from therelatively low dispersion glass and the additional material. A firstplurality of lenses is positioned on one side of a gap and a secondplurality of lenses is positioned on another side of the gap. The firstplurality of lenses causes an introduction of a significant amount ofcolor, and the gap and second plurality of lenses substantially cancelthe significant amount of color.

These and other aspects of the present invention will become apparent tothose skilled in the art from the following detailed description of theinvention and the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings in which:

FIG. 1 represents the problem of lateral color addressed by the presentdesign;

FIG. 2 is a 0.7 NA design using FPL51 for the positive elements, wherelateral color from 488 to 720 nm is less than 1 nm at the edge of a 50micron diameter field;

FIG. 3 shows a 0.8 NA design using FPL51 glass for the positiveelements, where lateral color from 488 to 720 nm is less than 1 nm atthe edge of a 50 micron diameter field;

FIG. 4 illustrates a 0.8 NA design using FPL53 for the positiveelements, where the FPL53 glass has a lower dispersion than the FPL51glass, and lateral color from 488 to 720 nm is less than 1 nm at theedge of a 50 micron diameter field; and

FIG. 5 is a further 0.8 NA design using FPL53 for the positive elements,where lateral color in the wavelength range from 488 to 720 nm is lessthan 1 nm at the edge of a 50 micron diameter field.

DETAILED DESCRIPTION OF THE INVENTION

The present design employs a new method to reduce the amount of lateralcolor in an optical system. Lateral color is created within an opticalsystem because of the natural dispersion properties of glass materialsused to make lenses. This dispersion causes different wavelengths toform images at different locations and having different amounts ofmonochromatic aberrations. This dispersion creates a shift in the imagecentroid position for each wavelength. Using this new method, a lens maybe provided within the design to produce a significant amount of color,and subsequently canceling the color using a relatively large airspacebetween the first lens and another lens with a controlled amount ofcolor. In this manner, lateral color effects can be reduced and, inparticular, centroid shifting due to different colors can be minimized.

Many refractive optical designs have been developed having a broadspectral range or band for the illumination employed. Typical inspectionsystems employ an illuminator transmitting illumination energy at one ormore wavelengths, and optics generally designed to particularly focusand efficiently utilize the transmitted illumination energy. Manydifferent illumination sources, including but not limited to broadbandlight sources and lasers have been employed. When the illuminators andother inspection components exhibit shorter focal lengths, such as inthe case of microscope objectives, color aberrations can become verylimiting to the lens resolution when covering a broad spectral band.

If the highest possible color and aberration correction is required,over a broad band, four general design principles may be employed.

The first design principle minimizes the color present inside thesystem, where inside the system represents within the optical componentsused to inspect the specimen. Color minimization entails providing thesmallest possible amount of color, thereby minimizing the lateral colorcorrection required. Color minimization occurs when most of the positivepower in the design, especially near the aperture stop, is constructedfrom a very low dispersion glass—ideally calcium fluorite, FPL51, FPL53or a close approximation to such a low dispersion glass type. Suchglasses are generally available from high precision lens manufacturersand are-known by these designations to those skilled in the art. Use ofglasses, lenses, and/or optical components of this type provides arelatively small amount of color that may then be corrected by othercomponents within the design.

The second design principle is to correct the minimized color presentusing a different glass that matches very well with respect to thehigher-order dispersion properties of the first glass type. If thisdesign principle is followed, the secondary and higher order colorresiduals will generally be relatively small. For example, for FPL51glass, a very good match exists with respect to higher-order dispersionproperties in silica. Thus using negative lenses fabricated from silicamay beneficially correct the color of the positive lenses, such as thoseof Ohara FPL51 glass, and may provide very little residual higher-ordercolor.

Use of different matched glasses in this manner can sufficiently correctcolor for positive lenses. However, in high NA designs, a minimaldispersion difference exists between FPL51 glass and silica, thusrequiring certain relatively strong power lenses with steep radii inorder to obtain adequate color correction. In a high NA design, totalinternal reflection and adverse monochromatic aberrations tend to beprevalent. Both of these problems may be diminished if the negativesilica lenses are attached to the positive FPL51 lenses, such as byusing cement or other adhesive. The result of cementing lenses, forexample, may be that the cemented interface between these two low indexglasses does not provide a significant amount of monochromaticaberration correction as the two glass index values are too similar. Thepresent design addresses monochromatic aberration issues by using othernon-joined lenses perform monochromatic aberration correction. Usingother non-joined lenses can increase the complexity of the design.

An alternative method calls for the negative lenses in the design toexhibit a slightly higher index than the FPL51 positive lenses. Thisallows the design to achieve some monochromatic aberration correction atthe cemented interface. Monochromatic aberration correction at thecemented interface results in the FPL51 positive lenses not having colorcorrected by the silica glass. Negative lenses used in the presentdesign, typically constructed of Ohara BSL7, tend to be a good match inhigher-order dispersion properties to FPL51, but not a perfect match.The result is that over a very broad spectral range, the design mayexhibit large residual color aspects that may be unacceptable in manyapplications.

The third design principle employs the “dense-flint” principle tocorrect higher-order color aberrations. The dense-flint principleinvolves the use of a relatively small amount of lens power with a glassthat has a very different higher-order dispersion property from theprevious two glasses, where the previous two glasses are well matchedin. higher order dispersion characteristics. Usually the glass havingthe different higher-order dispersion property is a very high indexflint glass that is highly dispersive. In the present design, oneembodiment may include the glass Ohara TIH 11. The use of a small amountof power of this very different higher-order dispersion properties glassin the design allows for a very beneficial enhancement of residual coloraberrations and can give correction for secondary and even tertiarycolor effects. Use of a glass such as a very high index flint glass cancorrect higher-order lateral color effects if the glass is positioned inthe design at a relatively remote distance from the aperture stop.Positioning the glass at a relatively remote distance from the aperturestop can control higher-order paraxial lateral color to compensate forchromatic variation of coma. Coma is a type of lateral color thatdepends on the aperture and is not paraxial.

The new design principle used in these embodiments to correct themajority of the residual centroid based lateral color is to introduce arelatively large amount of color in the middle section of the designusing either a positive or negative lens or lens group and then cancelthe large amount of color using a relatively large airspace between thecomponents that put in the color and those that subsequently cancel thecolor out. These subsequent components have a total power that is of theopposite sign to the group producing the color. Inserting an abundanceof color between separated components makes for different ray heightsand angles for each color. By the time these rays reach the secondoptical or downstream component group, the result is a chromaticvariation in spherical aberration and coma. Chromatic variationsrepresent an induced effect due to aberrations between designcomponents. These chromatic variations are not an effect substantiallyintrinsic to the surfaces. In very highly corrected designs, theseinduced effects can be as important as aberrations intrinsic to thesurfaces and can thus materially affect the resultant image or images.In the present design, the introduction of significant color in thelarge middle airspace of the design allows the resulting inducedchromatic variation of spherical aberration and coma to correct for theintrinsic aberrations of that type using the rest of the components inthe design. The result is a very highly corrected high NA design, withextremely good performance.

If system length is not of significant concern, then the relatively longairspace in the middle of the design could be nearly collimated and astretched-out design could be very well corrected using the foregoingdesign principles. But if a short design is required, the airspace inthe middle of the design must be in fairly strongly converging light.

Other designs exhibiting similar correction tendencies may be achievableaccording to the foregoing principles, but the results are generallysensitive to the glass choices.

Various examples of designs adhering to the foregoing principles areillustrated in FIGS. 2-4. FIG. 2 is a 0.7 NA design using FPL51 for thepositive elements. The lateral color from 488 to 720 nm is less than 1nm at the edge of a 50 micron diameter field.

TABLE 1 Prescription for the design of FIG. 2. Radius Thickness GlassOBJ Infinity Infinity  1 −3.45081 1.1 S-FPL51  2 4.83798 1.5  3 −15.94671.74991 S-TIH11  4 −4.88786 4.729387 S-FPL51  5 −6.4339 0.25  6 51.14521.1 BAM21  7 7.238453 4.999964 S-FPL51  8 −7.45296 4.181398  9 −6.94461.1 BAM21 10 4.811282 4.5 S-FPL51 11 −37.7316 0.109924 12 20.77116 4.5S-FPL51 13 −4.82976 1.1 S-BSL7 14 3905.813 0.101135 STO Infinity 0.11 1610.95898 4.5 S-FPL51 17 −5.27083 1.1 S-BSL7 18 −40.9304 0.11 19 5.9281533.75 S-FPL51 20 −5.27979 1.1 S-BSL7 21 1338.503 0.11 22 2 2.459581S-FPL51 23 2.704 0.500003 IMA Infinity

The foregoing table reads the various components in FIG. 2 from left toright. As may be appreciated by one skilled in the art, the numbers inthe leftmost column of Table 1 represent the surface number countingsurfaces from the left of FIG. 2. For example, the left-surface of lens201 in the orientation presented in FIG. 2 (surface 1 in Table 1) has araduis of curvature of −3.45081 mm, a thickness of 1.1 mm, and therightmost surface (surface 2) has a radius of curvature of 4.83798 mm,and is 1.5 mm from the next surface. The material used is S-FPL51, orFPL51. The designation STO represents the aperture stop, IMA the image,and OBJ the objective.

Stepping through the lenses of the objective design 200 of FIG. 2, fromleft to right, first lens 201 is constructed of FPL51, while second lens202 is formed from flint glass TIH 11. Third lens 203 is formed ofFPL51. Fourth lens 204 is formed of BAM21, while fifth lens 205 isconstructed of FPL51. Gap 250 is formed between fifth lens 205 and sixthlens 206, where sixth lens 206 is formed from BAM21. Seventh lens 207 isconstructed of FPL51, as is eighth lens 208. Ninth lens 209 is formedfrom BSL7, while tenth lens 210 uses FPL51, eleventh lens 211 BSL7,twelfth lens 212 FPL51, thirteenth lens 213 BSL7, and fourteenth lens214 FPL51. Aperture stop 220 is shown between ninth lens 209 and tenthlens 210. Note that the net power in the first plurality of lenses,namely first lens 201 through fifth lens 205, is opposite in sign fromthe second plurality of lenses, namely sixth lens 206 through fourteenthlens 214. For example, the first plurality may be positive while thesecond is negative, or vice versa.

The present design employs optical cement where two elements are incontact with one another. In the embodiment of FIG. 2, cement isprovided between lens elements 202 and 203, lens elements 204 and 205,lens elements 206 and 207, lens elements 208 and 209, lens elements 210and 211, and lens elements 212 and 213. UV cure epoxies speciallydeveloped for this application are typically used, but other types ofcement are possible.

In this construction, the four principles noted earlier apply asfollows. The first principle, color minimization, occurs when most ofthe positive power in the design, especially near the aperture stop, isconstructed from a very low dispersion glass. As shown in FIG. 2,seventh, eighth, tenth, and twelfth lenses 207, 208, 210, and 212 arepositioned near the aperture stop, and seventh lens 207, eighth lens208, tenth lens 210, and twelfth lens 212 are constructed from lowdispersion FPL51. The design of FIG. 2 also corrects the color presentusing a different glass that matches very well with respect to thehigher-order dispersion properties of the first glass type. Here thefirst glass type is FPL51, and the matching glass is BSL7, again amaterial known to those skilled in the precision glassmaking art andfamiliar with Ohara types and Ohara codes. The design of FIG. 2 furtheremploys dense flint principle, the third principle noted above, wherethe dense flint glass TIH11 is used in lens 202. Finally, the design ofFIG. 2 introduces a relatively large amount of color in the middlesection of the design, and then cancels the large amount of color usinga relatively large airspace between the components that insert the colorand those that subsequently cancel the color out. Gap 250 separateslenses that introduce color, namely lenses 201-205, from those thatremove the color, generally lenses 206-214. In this manner, a designthat minimizes lateral color issues may be

FIG. 3 is a 0.8 NA design using FPL51 glass for the positive elements,again reflecting the four foregoing principles. The lateral color in thewavelength range from 488 to 720 nm is less than 1 nm at the edge of a50 micron diameter field.

TABLE 2 Prescription for the design of FIG. 3. Surf Radius ThicknessGlass OBJ Infinity Infinity  1 −2.640033 1.1 S-FPL51  2 6.225586 1.5  3−3.883463 1.499939 5-TIH11  4 −3.196875 1.1 S-FPL51  5 −6.06235 0.11  617.79942 4.55255 S-FPL51  7 −8.693053 0.11  8 34.32157 1.1 BAM21  95.894936 3.999935 S-FPL51 10 −9.132433 4.52315 11 −6.94491 1.1 BAM21 124.646253 3.500181 S-FPL51 13 578.2921 0.561037 14 23.67839 4.27221S-FPL51 15 −4.621956 1.1 S-BSL7 16 1993.301 0.102457 STO Infinity 0.1118 10.09419 3.749717 S-FPL51 19 −5.356311 1.1 S-BSL7 20 47.258490.1099999 21 7.107647 3.75 S-FPL51 22 −5.312922 1.1 S-BSL7 23 33.190320.11 24 3.66806 1.5 S-FPL51 25 8.445074 0.11 26 2 2.297492 S-FPL51 272.040446 0.5000674 IMA Infinity

From FIG. 3, objective 300 includes first lens 301 which is formed ofFPL51, second lens 302 is formed from flint glass TIH11, third lens 303from FPL51, and fourth lens 304 from FPL51. Fifth lens 305 isconstructed from BAM21, sixth lens 306 from FPL51, seventh lens 307 fromBAM21, eighth lens 308 from FPL51, and ninth lens 309 from FPL51, andthe tenth lens 310 from BSL7. Aperture stop 320 is provided, and gap 350is provided between sixth lens 306 and seventh lens 307. Eleventh lens311 is constructed from FPL51, Twelfth lens 312 from BSL7, thirteenthlens 313 from FPL51, and Fourteenth lens 314 from BSL7. Fifteenth lens315 is formed from FPL51, and sixteenth lens 316 is formed from FPL51Again the lenses that are in contact, namely lens 302 and 303, lenses305 and 306, lenses 307 and 308, lenses 309 and 310, lenses 311 and 312,and lenses 313 and 314 are cemented together.

FIG. 4 is a 0.8 NA design using FPL53 for the positive elements. Thisglass has a lower dispersion than the FPL51 glass. The lateral color inthe wavelength range from 488 to 720 nm is less than 1 nm at the edge ofa 50 micron diameter field.

TABLE 3 Prescription for the design of FIG. 4. Surf Radius ThicknessGlass OBJ Infinity Infinity  1 −2.70283 1.1 S-FPL53  2 5.399748 1.5  3−3.44088 2.509086 S-TIH11  4 −4.46521 0.495999  5 17.47191 2.974508S-FPL53  6 −15.0158 0.11  7 19.85473 1.1 BAM4  8 6.98409 4.00001 S-FPL53 9 −7.98404 6.914389 10 −6.78548 1.1 BAM4 11 4.448233 3.25 S-FPL53 12−31.1679 0.110758 13 93.80093 3.5 S-FPL53 14 −4.38537 1.1 FSL3 15−153.15 0.099999 STO Infinity 0.11 17 11.09318 4 S-FPL53 18 −5.03763 1.1FSL3 19 −38.0933 0.11 20 7.069142 3.75 S-FPL53 21 −5.55846 1.1 FSL3 22−177.003 0.11 23 3.597066 1.5 S-FPL53 24 9.624745 0.11 25 2 2.432919S-FPL53 26 1.719818 0.49998 IMA Infinity

From FIG. 4, objective 400 comprises first lens 401 formed of FPL53.Second lens 402 is formed from flint glass TIH11, third lens 403 fromFPL53, and fourth lens 404 from BAM4. Fifth lens 405 is constructed fromFPL53, sixth lens 406 from BAM4, seventh lens 407 from FPL53, eighthlens 408 from FPL53, and ninth lens 409 from FSL3. Aperture stop 420 isprovided, and gap 450 is provided between fifth lens 405 and sixth lens406. Tenth lens 410 is constructed from FPL53, eleventh lens 411 fromFSL3, twelfth lens 412 from FPL53, and thirteenth lens 413 from FSL3.Fourteenth lens 414 and fifteenth lens 415 are both formed from FPL53.Again, this design follows the four foregoing principles, using lowdispersion glass FPL53, matching glass FSL3, flint glass TIH11, andproviding gap 450, such that lenses 401-405 introduce significant colorwhile lenses 406-415 remove the color.

FIG. 5 is a further 0.8 NA design using FPL53 for the positive elements.This glass has a lower dispersion than the FPL51 glass. The lateralcolor in the wavelength range from 488 to 720 nm is less than 1 nm atthe edge of a 50 micron diameter field.

TABLE 4 Prescription for the design of FIG. 5. Surf Radius ThicknessGlass OBJ Infinity Infinity  1 3.295736 1.749972 S-BSL7  2 7.600172 1  3−26.95779 1.1 S-FPL53  4 1.84435 1.5  5 −2.251053 6.687232 S-FPL53  6−6.093807 3.723266  7 47.76675 1.75 S-TIH11  8 −23.67633 0.5  9−31.96948 1.1 S-BAM4 10 7.165765 3 S-FPL53 11 −10.76491 0.5 12 −23.644241.1 S-BAM4 13 7.45945 2.5 S-FPL53 14 −108.5253 0.25 15 20.66576 3S-FPL53 16 −6.61093 1.1 FSL3 17 Infinity 0.25 STO Infinity 0.11 1912.09707 3.25 S-FPL53 20 −6.469096 1.1 FSL3 21 −2134.371 0.1099999 228.354132 3.75 S-FPL53 23 −6.302046 1.1 FSL3 24 −42.86506 0.11 253.734808 1.75 S-FPL53 26 8.546036 0.11 27 1.8 2.249545 S-FPL53 281.492067 0.5 IMA Infinity

From FIG. 5, objective 500 comprises first lens 501 formed of BSL7.Second lens 502 is formed from flint FPL53, third lens 503 from FPL53,and fourth lens 504 from flint glass TIH11. Fifth lens 505 isconstructed from BAM4, sixth lens 506 from FPL53, seventh lens 507 fromBAM4, eighth lens 508 from FPL53, and ninth lens 509 from FPL53.Aperture stop 520 is provided, and gap 550 is provided between thirdlens 503 and fourth lens 504. Tenth lens 510 is constructed from FSL3,eleventh lens 511 from FPL53, twelfth lens 512 from FSL3, and thirteenthlens 513 from FPL53 and the fourteenth lens from FSL3. Fifteenth lens515 and sixteenth lens 516 are both formed from FPL53. Again, thisdesign follows the four foregoing principles, using low dispersion glassFPL53, matching glass FSL3, flint glass TIH11, and providing gap 550,such that lenses 501-503 introduce significant color while lenses504-515 remove the color. This design demonstrates using a negative lensgroup consisting of elements 501 through elements 503 introducing colorthat is compensated by positive lens group 504-515.

The present design may be employed in various environments, includingbut not limited to semiconductor wafer inspection/lithography,biological inspection, medical research, and the like.

While the invention has been described above by reference to certainembodiments, it will be understood that changes and modifications may bemade without departing from the scope of the invention, which is to bedefined only by the appended claims and their equivalents. For example,while the embodiments are illustrated with respect to specific lensingarrangements conforming to certain principles, the invention may beconstructed in other ways and used in inspecting various types ofspecimens, including but not limited to semiconductor masks, wafers andother lithography applications, biological specimens, and so forth.While the invention has thus been described in connection with specificembodiments thereof, it will be understood that the invention is capableof further modifications. This application is intended to cover anyvariations, uses or adaptations of the invention following, in general,the principles of the invention, and including such departures from thepresent disclosure as come within known and customary practice withinthe art to which the invention pertains.

1. A method for providing color correction of a specimen beinginspected, comprising: forming an objective comprising multiple lensesformed from different materials; and positioning a first plurality oflenses comprising a positive lens on one side of a gap in the objectiveand a second plurality of lenses comprising a negative lens on anotherside of the gap, wherein the first plurality of lenses inserts asignificant amount of color, and the gap and second plurality of lensessubstantially cancel the significant amount of color; wherein dimensionsof the first plurality of lenses, gap, and second plurality of lensesare selected to reduce lateral color encountered during inspection. 2.The method of claim 1, wherein the multiple lenses formed from differentmaterials comprise at least one lens formed from a relatively lowdispersion glass.
 3. The method of claim 2, wherein the multiple lensesformed from different materials further comprise at least one additionallens from an additional material different from the relatively lowdispersion glass of the at least one lens.
 4. The method of claim 3,wherein the multiple lenses formed from different materials furthercomprises at least one further lens from a further material differentfrom the relatively low dispersion glass and the additional material. 5.The method of claim 4, wherein the further material provides arelatively small amount of lens power and exhibits a significantlydifferent dispersion power from the relatively low dispersion glass andthe additional material.
 6. The method of claim 2, wherein therelatively low dispersion glass comprises one from a group comprisingcalcium fluorite, FPL51, and FPL53.
 7. The method of claim 3, whereinthe additional material comprises silica.
 8. The method of claim 4,wherein the further material comprises a highly dispersive high indexflint glass.
 9. An objective for inspecting a specimen, comprising:multiple lenses constructed from different materials, the multiplelenses comprising: a first plurality of lenses comprising a positivelens, the first plurality of lenses positioned on one side of a gap inthe objective; and a second plurality of lenses comprises a negativelens and is positioned on another side of the gap; wherein the firstplurality of lenses causes an introduction of a significant amount ofcolor within the objective, and the gap and second plurality of lensessubstantially cancel the significant amount of color and dimensions ofthe first plurality of lenses, gap, and second plurality of lenses areselected to reduce lateral color effects encountered during inspection.10. The objective of claim 9, wherein the multiple lenses constructedfrom different materials comprise: at least one lens constructed from arelatively low dispersion glass; at least one additional lensconstructed from an additional material different from the relativelylow dispersion glass of the at least one lens, wherein the additionalmaterial generally matches low dispersion properties of the relativelylow dispersion glass of the at least one lens; and at least one furtherlens constructed from a further material different from the relativelylow dispersion glass and the additional material.
 11. The objective ofclaim 10, wherein the further material provides a relatively smallamount of lens power and exhibits a significantly different dispersionpower from the relatively low dispersion glass and the additionalmaterial.
 12. The objective of claim 10, wherein the relatively lowdispersion glass comprises one from a group comprising calcium fluorite,FPL51, and FPL53.
 13. The objective of claim 12, wherein the additionalmaterial comprises silica.
 14. The objective of claim 13, wherein thefurther material comprises a highly dispersive high index flint glass.15. An objective, comprising: a plurality of lenses constructed from atleast one predetermined lens material, the plurality of lensescomprising: a first plurality of lenses comprising at least one positivelens, the first plurality of lenses positioned on one side of a gapformed in the objective; and a second plurality of lenses comprising atleast one negative lens positioned on another side of the gap; whereinthe first plurality of lenses causes an introduction of a significantamount of color into the objective, and the gap and second plurality oflenses substantially cancel the significant amount of color, anddimensions of the first plurality of lenses, gap, and second pluralityof lenses are selected to reduce lateral color encountered by theobjective.
 16. The objective of claim 15, wherein the plurality oflenses comprises: at least one lens constructed from a relatively lowdispersion glass; at least one additional lens constructed from anadditional material matched with and different from the relatively lowdispersion glass; and at least one further lens constructed from afurther material different from the relatively low dispersion glass andthe additional material.
 17. The objective of claim 16, wherein thefurther material provides a relatively small amount of lens power andexhibits a significantly different dispersion power from the relativelylow dispersion glass and the additional material.
 18. The objective ofclaim 16, wherein the relatively low dispersion glass comprises one froma group comprising calcium fluorite, FPL51, and FPL53.
 19. The objectiveof claim 18, wherein the additional material comprises silica.
 20. Theobjective of claim 19, wherein the further material comprises a highlydispersive high index flint glass.
 21. The objective of claim 16,wherein the first plurality of lenses is opposite in sign from thesecond plurality of lenses.